Bennu is the target of the OSIRIS-REx mission which is intended to return samples to Earth in 2023 for further study.[16][17][18] On 3 December 2018, the OSIRIS-REx spacecraft arrived at Bennu after a two-year journey.[19] Before attempting to obtain a sample from the asteroid, it will map out Bennu's surface in detail and orbit the asteroid to calculate its mass.[20]

Physical description

This mosaic image of asteroid Bennu is composed of 12 PolyCam images collected on Dec. 2 by the OSIRIS-REx spacecraft from a range of 15 miles (24 km).

Asteroid Bennu has a roughly spheroidal shape, resembling a spinning top. The direction of rotation about its axis is retrograde with respect to its orbit. Bennu has a fairly smooth shape with one prominent 7001100000000000000♠10–20 m boulder on its surface, in the southern hemisphere.[13]

There is a well-defined ridge along the equator of asteroid Bennu. The presence of this ridge suggests that fine-grained regolith particles have accumulated in this area, possibly because of its low gravity and fast rotation.[13]

Observations of this minor planet by the Spitzer Space Telescope in 2007 gave an effective diameter of 7002484000000000000♠484±10 m, which is in line with other studies. It has a low visible geometric albedo of 6998460000000000000♠0.046±0.005. The thermal inertia was measured and found to vary by ±19% during each rotational period. The data suggest that the regolith grain size is moderate, ranging from several millimeters up to a centimeter, and evenly distributed. No emission from a potential dust coma has been detected around asteroid Bennu, which puts a limit of 106 g of dust within a radius of 4750 km.[21]

Astrometric observations between 1999 and 2013 have demonstrated that 101955 Bennu is influenced by the Yarkovsky effect, causing the semimajor axis to drift on average by 7002284000000000000♠284±1.5 meters/year. Analysis of the gravitational and thermal effects has given a bulk density of ρ = 7003126000000000000♠1260±70 kg/m3, which is only slightly denser than water. Therefore, the predicted macroporosity is 7001400000000000000♠40±10%, suggesting that the interior has a rubble pile structure. The estimated mass is 7010780000000000000♠(7.8±0.9)×1010 kg.[6]

Photometry and spectroscopy

Photometric observations of Bennu in 2005 yielded a synodic rotation period of 7004154458000000000♠4.2905±0.0065 h. It has a B-type classification, which is a sub-category of carbonaceous asteroids. Polarimetric observations show that Bennu belongs to the rare F subclass of carbonaceous asteroids, which is usually associated with cometary features.[9] Measurements over a range of phase angles showed a phase function slope of 0.040 magnitudes per degree, which is similar to other near-Earth asteroids with low albedo.[22]

Preliminary spectroscopic surveys of the asteroid's surface by OSIRIS-REx spacecraft, detected the presence of hydrated minerals in the form of clay. While researchers suspect that Bennu was too small to host water, the hydroxyl groups may have come from water presence in its parent body before Bennu split off.[23][24]

Asteroid Bennu's basic mineralogy and chemical nature would have been established during the first 10 million years of the Solar System's formation, where the carbonaceous material underwent some geologic heating and chemical transformation into more complex minerals.[13] Bennu probably began in the inner asteroid belt as a fragment from a larger body with a diameter of 100 km. Simulations suggest a 70% chance it came from the Polana family and a 30% chance it derived from the Eulalia family.[26]

Subsequently, the orbit drifted as a result of the Yarkovsky effect and mean motion resonances with the giant planets, such as Jupiter and Saturn. Various interactions with the planets in combination with the Yarkovsky effect modified the asteroid, possibly changing its spin, shape, and surface features.[27]

Cellino et al. have suggested a possible cometary origin for Bennu, based on similarities of its spectroscopic properties with known comets. The estimated fraction of comets in the population of Near Earth asteroids is 6998800000000000000♠8%±5%.[9]

Possible Earth impact

On average, an asteroid with a diameter of 500 m (1,600 ft; 0.31 mi) can be expected to impact Earth about every 130,000 years or so.[28] A 2010 dynamical study by Andrea Milani and collaborators predicted a series of eight potential Earth impacts by Bennu between 2169 and 2199. The cumulative probability of impact is dependent on physical properties of Bennu that were poorly known at the time, but was found to not exceed 0.071% for all eight encounters.[29] The authors recognized that an accurate assessment of 101955 Bennu's probability of Earth impact would require a detailed shape model and additional observations (either from the ground or from spacecraft visiting the object) to determine the magnitude and direction of the Yarkovsky effect.

The publication of the shape model and of astrometry based on radar observations obtained in 1999, 2005, and 2011,[4] made possible an improved estimate of the Yarkovsky acceleration and a revised assessment of the impact probability. The current (as of 2014)[update] best estimate of the impact probability is a cumulative probability of 0.037% in the interval 2175 to 2196.[6] This corresponds to a cumulative score on the Palermo scale of −1.71. If an impact were to occur, the expected kinetic energy associated with the collision would be 1,200 megatons in TNT equivalent (for comparison, TNT equivalent of Little Boy was approx 15 kiloton).[5]

2060

Bennu will pass 0.005 au (750,000 km; 460,000 mi) from Earth on 23 September 2060.[2] The close approach of 2060 causes divergence in the close approach of 2135. On 25 September 2135, the nominal approach distance is 0.002 au (300,000 km; 190,000 mi) from Earth, but Bennu could pass as close as 0.0007 au (100,000 km; 65,000 mi).[2] There is no chance of an Earth impact in 2135.[30][5] The 2135 approach will create many lines of variations and Bennu may pass through a gravitational keyhole during the 2135 passage which could create an impact scenario at a future encounter. The keyholes are all less than 55 km wide.[6]

On 25 September 2175, there is a 1 in 24,000 chance of an Earth impact,[5] but the nominal 2175 approach is in February 2175 at a distance of roughly 0.1 au (15,000,000 km; 9,300,000 mi).[2] The most threatening virtual impactor is on 24 September 2196 when there is a 1 in 11,000 chance of an Earth impact.[5] There is a cumulative 1 in 2,700 chance of an Earth impact between 2175–2199.[5]

Long term

Lauretta et al. reported in 2015 their results of a computer simulation, concluding that it is more likely that 101955 Bennu will be destroyed by some other cause:

The orbit of Bennu is intrinsically dynamically unstable, as are those of all NEOs. In order to glean probabilistic insights into the future evolution and likely fate of Bennu beyond a few hundred years, we tracked 1,000 virtual "Bennus" for an interval of 300 Myr with the gravitational perturbations of the planets Mercury–Neptune included. Our results ... indicate that Bennu has a 48% chance of falling into the Sun. There is a 10% probability that Bennu will be ejected out of the inner Solar System, most likely after a close encounter with Jupiter. The highest impact probability for a planet is with Venus (26%), followed by the Earth (10%) and Mercury (3%). The odds of Bennu striking Mars are only 0.8% and there is a 0.2% chance that Bennu will eventually collide with Jupiter.[27]

OSIRIS-REx

Animation of OSIRIS-REx's trajectory from 9 September 2016 to 3 December 2018.OSIRIS-REx; 101955 Bennu; Earth; Sun;

Animation of OSIRIS-REx's trajectory around 101955 Bennu from 26 December 2018 to 20 March 2021OSIRIS-REx· 101955 Bennu

The OSIRIS-REx mission of NASA's New Frontiers program was launched towards 101955 Bennu on September 8, 2016. On December 3, 2018, the spacecraft arrived at the asteroid Bennu after a two-year journey.[19] One week later, at the American Geophysical Union Fall Meeting, investigators announced that OSIRIS-REx had discovered spectroscopic evidence for hydrated minerals on the surface of the asteroid, implying that liquid water was present in Bennu’s parent body.[31] OSIRIS-REx is expected to return samples to Earth in 2023.[32]

Selection

Bennu was selected from over 7005500000000000000♠500000 known asteroids by the OSIRIS-REx selection committee. The primary constraint for selection was close proximity to Earth, since proximity implies low impulse (Δv) required to reach an object from Earth orbit.[33] The criteria stipulated an asteroid in an orbit with low eccentricity, low inclination, and an orbital radius of 7011119678296560000♠0.8–1.6 au.[34] Furthermore, the candidate asteroid for a sample-return mission must have loose regolith on its surface, which implies a diameter greater than 200 meters. Asteroids smaller than this typically spin too fast to retain dust or small particles. Finally, a desire to find an asteroid with pristine carbon material from the early Solar System, possibly including volatile molecules and organic compounds, reduced the list further.

With the above criteria applied, five asteroids remained as candidates for the OSIRIS-REx mission, and Bennu was chosen, in part for its potentially hazardous orbit.[34]

Name

The name Bennu was selected from more than eight thousand student entries from dozens of countries around the world who entered a "Name That Asteroid!" contest run by the University of Arizona, The Planetary Society, and the LINEAR Project.[2][11] Third-grade student Michael Puzio from North Carolina proposed the name in reference to the Egyptian mythological bird Bennu. To Puzio, the OSIRIS-REx spacecraft with its extended TAGSAM arm resembles the Egyptian deity, which is typically depicted as a heron.[2]

Gallery

The orbits of asteroid 101955 Bennu and the four inner planets around the Sun

A compilation of radar images of asteroid Bennu (left) and a corresponding 3D shape model (right)

OSIRIS-REx's first images of asteroid Bennu

This picture, taken by the OSIRIS-REx spacecraft on November 2nd 2018, was part of a sequence of frames collected to show asteroid 101955 Bennu rotating. Bennu is approximately 200 pixels wide in this shot.